1. Field of the Invention
The invention relates generally to wireless audio transmission apparatuses. More particularly, it relates to an improved Frequency Modulation (FM) output portable audio player for digitally coupling audio signals generated therein to be broadcasted through an automobile's radio receiver system.
2. Description of the Prior Art
As is generally known in the art, portable audio players using MP3 or other compression have been very popular to consumers in the digital audio environment. In using of such portable audio players, there are provided suitable controls on the audio player which enables digitally compressed audio sounds, such as music, that have been stored on a memory card to be played through headphones attached thereto by a cable.
While the headphones can be easily used in connection with a number of activities associated with the portable audio players, the use of headphones when driving an automobile has been however generally prohibited by law as creating an unsafe driving condition. As a result, there have been developed heretofore in the prior art portable audio players which can be used in an automobile where the compressed audio sounds converted from the digitally stored music in the audio players are broadcasted through an automobile radio receiver system. One such prior art portable audio player is shown in FIG. 1 and has been labeled as “Prior Art”.
As depicted in FIG. 1, there is illustrated in block diagram form a traditional audio system 10, suitable for illustrating one of the prior art systems. In particular, the audio system 10 includes a portable audio player 12, a wireless audio link 14, and a remote audio receiver 16. The portable audio player 12 may be in the form of a CD, MP3, DVD player or the like carrying a digital audio storage media which is capable of generating stereo audio signals. In particular, the MP3 player 12 includes an audio signal source, such as a microprocessor 18, for generating digital audio output signals on lines 20 and 22 and a digital-to-analog (D/A) converter 24 responsive to the digital signals for providing stereo left and right analog outputs on respective conductors 26 and 28.
The wireless audio link 14 is suitably disposed in a cradle housing (not shown) and contains appropriate circuitry which is electrically connected to the analog outputs on the conductors 26 and 28 of the MP3 player 12 for wirelessly transmitting the audio signals on a RF carrier signal to the remote audio receiver system 16. The wireless audio link 14 includes a FM stereo multiplexer 30 and a phase-locked-loop (PLL) network 32, RF amplifier 34, and antenna 36. A pair of cables or conductors 38 and 40 having respective jacks 42, 44 extending from the cradle housing of the audio link 14 is connected to the conductors 26, 28 via the corresponding left and right audio output/headphones jacks 46, 48 of the MP3 player.
In addition to receiving the stereo left and right analog outputs via the respective jacks 42, 44, the FM stereo multiplexer 30 receives on line 50 a 38 kHz carrier used to modulate a left-minus-right (L-R) signal information which is then centered at 38 kHz and receives on line 52 a pilot tone located at 19 kHz. The FM stereo multiplexer 30 generates a composite multiplexed audio signal on its output line 54.
The PLL network 32 includes a loop filter 56 whose output on line 57 is fed to a first input of a summing circuit 58. The summing circuit 58 receives as a second input the composite multiplexed audio signal on the line 54 from the FM multiplexer 30. The output on line 60 of the summing circuit 58 is passed to a voltage-controlled oscillator (VCO) 62. The output of the VCO on line 64 is passed through a digital/frequency divider 66 and a phase (frequency) detector 68 to the input of the loop filter 56. The ratio of the digital/frequency divider 66 serves to adjust the transmission frequency of the FM transmitter in order to select a station channel. The phase detector 68 also receives a reference clock as a second input on line 69. The output of the VCO 62 is also fed to the RF amplifier 34 where the signal on the output thereof is transmitted via the antenna 36.
The transmitted signal is then received by the audio receiver system 16 via an antenna 17. The audio receiver 16 consists of a FM stereo receiver 19 which may be located in an in-house stereo receiver system, an in-car mounted FM radio receiver or the like. The radio receiver 19 is provided with stereo left and right channel speakers 70 and 72, respectively. The FM transmitted signals on the RF carrier signal are received by the FM stereo receiver 19 via the antenna 17 where the audio signals are demodulated and broadcasted over the automobile's stereo receiver 19.
Since all of the various electrical components in the cradle housing of the audio link 14 are quite large, they occupy a relatively large amount of space. This is due to the fact that the composite stereo signal is being generated in an analog format. Accordingly, it would therefore be desirable to generate the composite stereo audio signal in an entirely digital manner. This is advantageous since the digital processing circuitry can be fabricated with smaller integrated circuit chip(s), thereby reducing cost and space requirements.
The inventor of the present invention has developed a way of transmitting the composite stereo audio signal in a digital format so as to reduce the size of the electrical components and to obtain a higher audio quality than the audio system utilizing the analog format. These advantages are achieved through the provision of a wireless digital interface circuitry which is integrally mounted directly within the portable audio player. The wireless digital audio interface circuitry serves to replace the electrical components in the audio link 14 of FIG. 1. Further, the antenna 36 from the audio link 14 is also integrated into the portable audio player so as to provide a very compact size.
Another disadvantage in using the wireless audio link 14 is that the interface to the audio player 12 is still only through a small display located on the audio player 12. Since most new sound systems in automobiles today support radio data signals (RDS), the inventor has proposed to digitally multiplex the RDS data centered about a 57 kHz suppressed carrier along with the stereo analog signals. As a consequence, song data (artist name and song being played), radio station and call name, or any other interface data used by the portable audio player can be now viewed on a larger display located on the automobile's radio receiver.